I bought my steel belly bars in 1983 before pricing became extravagant. By 2010 the end of one had rusted off, where a telescoped section slid inside the main square-cross-section tube. Otherwise they outlived my camper.
For a temporary belly bar, what about this:
A 4"x4" treated lumber beam, long enough to stick out on either side (same way that metal bars do). Drill each end for a 3/8" or 1/2" diameter eye bolt, chain this to the bottom eye bolts of your camper.
This has to be at least as strong as the bottom connection of the camper, & I think would stand up for a long time. But I'm not an engineer.
I used this arrangement for a couple of weeks before my order for metal belly bars came through.
This should work very well in emergency situations, as mentioned here:
http://i44.photobucket.com/albums/f29/Jaxom44/Moab%20EJS%202014/IMG_0913.jpg.
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You can buy treated lumber beams most anywhere, cut & drill them with simple hand tools, no welding required.
I have not checked prices on these wood beams lately, wouldn't be surprised if they now cost more than metal ones!
Wire the beam to the frame to secure it when the tension is off, or use galvanized pipe strapping.
I later repurposed the beam when I paved my front drive for the first time -- used it as a threshold into the garage, whose floor was a couple of inches below the driveway grade, an error of the original builder I was trying to correct. The treated beam has been in the ground for 14 years & holding strong.